The invention relates to an improved process for preparing acrylic plexifilaments by flash extrusion of an aqueous dispersion of an acrylic polymer. The resulting acrylic polymer plexifilamentary strands have improved initial whiteness as well as improved whiteness retention on heating.
U.S. Pat. No. 3,774,387 (Woodell) teaches the preparation of acrylonitrile polymer plexifilamentary strands by extrusion of an aqueous dispersion of acrylonitrile polymer containing 25-40% by weight polymer under particular elevated temperature and pressure conditions. A plexifilament consists of an assembly of fibrils of irregular cross-section which are interconnected at various points to form a plexus. The fibrils tend to lie roughly parallel to the assembly. The fibrils may be thought of as an inter-mingled nonplanar matrix of very thin fiber or ribbon-like elements that are interconnected at various points to form a web-like three dimensional network or plexus.
To maintain uniformity of the Woodell aqueous dispersions, particulate water-insoluble stabilizers, comprising up to 15 percent (preferably 2 to 12 percent) based on weight of the polymer employed, may be used. Such stabilizers include: inorganic oxides, such as aluminum oxide; silicon compounds, such as colloidal silica, aluminum silicate, ethyl orthosilicate; cellulose; and cross-linked vinyl polymers, for example, having sulfonic acid groups. The dispersion may also be mechanically stirred to aid in maintaining uniformity. The dispersion is heated and then extruded at temperatures between about 260.degree. C. and about 280.degree. C. and pressures of between about 50 atmospheres and about 110 atmospheres (.about.5050-1150 kPa). The pH of the aqueous dispersion is maintained on the acidic side, usually by addition of sulfuric acid, and pH's of between 1.0 and 6.0, depending on the stabilizer present, are most useful. It is generally also useful when processing in batch equipment to hold the dispersion at its extrusion temperature for a short time before extruding, e.g., for about 1 to 5, 10, 15 or even 20 minutes; to assure conversion of all polymer particles to form the hydrate melt; and it is sometimes convenient to raise the temperature in two discrete and separate levels during the heating. Moreover, it is sometime advantageous to employ a pressure let-down region immediately adjacent the extrusion orifice. Excessive exposure of the polymer to high temperature in the presence of water should be avoided, however, since such exposure is adverse to plexifilament whiteness. If desired, a mixture of water and aceonitrile may be employed as the dispersant medium and lower extrusion temperatures (e.g., 220.degree. C.) may be used.
Use of particulate, water-insoluble stabilizers is disadvantageous but necessary when shaping a dispersion of polymer prepared in the usual way as a particulate slurry. Use of high temperatures in the presence of water gives rise to discoloration and hydrolysis of the acrylonitrile polymers known in the art.
Polymers for the preparation of acrylic fibers, which by definition contain 85% or more by weight acrylonitrile, are ordinarily prepared as an aqueous slurry using redox catalysts, e.g., potassium persulfate initiator and sodium bisulfite activator. In fiber form, these polymers have the disadvantage of being somewhat off-white in color as formed and discolor even further on heating at high temperatures. It is known that initial yellowness (lack of whiteness) and the tendency to discolor further on heating of the acrylic polymers is inversely related to the polymer molecular weight. Therefore, manufacturing practice has been to adjust polymer molecular weight to that required to provide fibers of acceptable whiteness. The use of higher molecular weight polymer than is needed to provide adequate fiber physical properties results in a loss of productivity since the solutions used in processing such polymers have higher viscosities than would otherwise be needed.
While the source of yellowness in acrylonitrile polymers and fibers prepared therefrom is not completely understood, it is now generally accepted that the color is due to a chromophoric structure consisting of a series of condensed naphthyridine rings each bearing a ##STR1## residue, several of which in an unbroken series absorb in the ultraviolet region of the spectrum, rendering the polymer yellow.
One method proposed for blocking formation of this chromophore is to prepare copolymers wherein the acrylonitrile units are separated by copolymeric units sufficiently often to prevent aggregation of the six or seven consecutive acrylonitrile units required for color formation. While effective, this method is generally not useful in the case of fibers because the amount of comonomer required, e.g., about 21% by weight in the case of methyl acrylate, is not conducive to good fiber properties, especially with respect to dimensional stability. Bulky comonomers are more effective on a weight percent basis in preventing formation of the chromophore but are equally disadvantages with respect to dimensional stability. For example, as little as 10.5 weight percent styrene copolymerized with 89.5% by weight acrylonitrile results in significant shrinkage of fibers prepared therefrom under the hot-wet conditions encountered in commercial dyeing and laundering. Most commercial acrylic fibers contain no more than 9% by weight comonomer(s).
It has recently been proposed by Brandrup, Peebles et al., Makromol. Chem., 98, 189 (1966) and Macromolecules, 1, 53-8 (1968) that the naphthyridine chromophores are formed from .beta.-ketonitrile groups derived from an adduct formed by free radical attack on the nitrile group in the polymer. U.S. Pat. No. 3,448,092 (Chiang) describes a polymerization process using coordination catalysts which provides acrylonitrile polymers having less than 5 .mu.eq./g. .beta.-ketonitrile groups. These polymers have improved stability to discoloration on heating. However, this process is disadvantageous because nonaqueous solvents must be used.
U.S. Pat. No. 3,828,013 (Nield) describes an emulsion polymerization process for preparing acrylonitrile polymers containing up to 95 mol percent acrylonitrile (90.6% acrylonitrile by weight when copolymerized with styrene) using a combination of low volatility and high volatility mercaptans as chain transfer agents to control molecular weight. Although primarily intended for the molding of bottles, the polymers are also said to be suitable for the preparation of fibers. Color stability of the polymers on heating is not mentioned.
Another emulsion polymerization process for the preparation of acrylonitrile polymers is described in U.S. Pat. No. 3,819,762 (Howe). Dodecyl mercaptan is used as a chain transfer agent in some of the examples but is not required by the claims. The resulting polymers containing up to 85% by weight acrylonitrile are suitable for molding into bottles. No suggestion is made that the polymers are suitable for the spinning of fibers.
The present invention provides an improved process for the preparation of acrylic polymer plexifilaments having the process advantages of reduced sensitivity to discoloration and hydrolysis of the polymer due to process interruptions, elimination of the need to use particulate, water-insoluble stabilizers and elimination of the need to isolate the polymer from its preparation mixture. The resulting plexifilaments have improved initial whiteness and improved whiteness retention on heating.
This invention provides an improved process for producing plexifilament strands of an acrylontrile polymer which comprises dispersing in water 25 to 45% by weight of an acrylonitrile polymer containing at least 91% by weight acrylonitrile units and up to 9% by weight copolymeric units having an intrinsic viscosity of 0.6 to 2.0, 7 to 23 .mu.eq./g. enolizable groups after mild acid treatment, 15 to 70 .mu.eq./g. thioether ends derived from a water insoluble mercaptan and less than 3.mu. eq./g. oxidizable hydrolysis fragments, heating the dispersion to a temperature of 200.degree. to 300.degree. C. while maintaining the dispersion under sufficient pressure to maintain the water in the liquid state, the time of such heating not exceeding about 30 minutes, and promptly flash-extruding the dispersion through an orifice into a region of substantially lower temperature and pressure to form a continuous strand of fibrillated plexifilaments. Preferably the intrinsic viscosity is 0.8 to 1.5 and most preferably the intrinsic viscosity is 0.9 to 1.1.
Polymer suitable for use in the present invention may be conveniently prepared as an aqueous emulsion using water, the desired monomers, relatively low concentrations of a free radical initiator, a surfactant and a water insoluble mercaptan as chain transfer agent. The resulting latex may be coagulated by any convenient means to facilitate isolation of the polymer.
The initiator may be a persulfate acid or salt such as potassium persulfate, an azo initiator such as azo-bis(isobutyronitrile), azo-bis(.alpha.,.alpha.-dimethylvaleronitrile) or azo-bis(.alpha.,.alpha.-dimethyl-.gamma.-methoxyvaleronitrile) or a peroxide initiator such as t-butyl peroxyneodecanoate or other free radical initiator known in the art.
Low radical concentration is achieved by using a low initiator concentration and operating at low monomer(s)/H.sub.2 O ratio and at temperatures as low as consistent with satisfactory conversion and yield. Usually polymerization in emulsion gives whiter, more stable polymer than polymerization in suspension, probably because the polymer accumulates in the non-aqueous phase and thus is insulated from attack by radicals which are formed in the aqueous phase from the water soluble initiator (persulfate). The dodecyl mercaptan or other thiol chain transfer agent serves a dual function. It controls molecular weight by end-capping growing polymer radicals with hydrogen while initiating another chain with the residual RS.radical. Not only is the hydrogen capped end of the first chain stable but also the thioether end of the new chain is highly stable. Thus the second function is to supply a preponderance of stable ends.
The mercaptan chain transfer agent should be essentially insoluble in water. Aliphatic mercaptans having more than 7 carbon atoms are essentially insoluble in water. Dodecyl mercaptan is preferred. Use of an essentially water insoluble mercaptan made available in the polymerization zone by addition of a mutual solvent or an effective emulsifier tends not only to increase the resistance of the polymer to discoloration but also to compensate for the lower polymerization rate entailed by using a low initiator concentration.
Although dodecyl mercaptan is the preferred chain transfer agent, other oil soluble mercaptans including alkyl or aralkyl mercaptans varying in carbon atoms per molecule from 6 to 20 or more may be used. Other nonreactive groups such as hydroxyls, ethers and esters may be present so long as they do not increase water solubility and decrease oil solubility greatly. A final consideration is that the shorter chain mercaptans of C.sub.8 or C.sub.6 carbon content typically give lower polymer yields than do longer chain mercaptans.
Suitable surfactants should be nonsubstantive on the polymer, i.e., other than cationic if the polymer is designed to be dyeable with cationic dyes. Approximately 5% by weight or less of this surfactant, based on monomers, should efficiently disperse the monomers and chain transfer agent and provide an emulsion of the polymer that is coagulable yet stable to monomer stripping conditions and storage. Preferably, the surfactant should be removable by washing with water. Alkylphenol polyethyleneoxy sodium sulfates having up to 10 ethyleneoxy groups are preferred. The corresponding phosphates are also useful but are more difficult to remove because of lower solubility in hot water. In most instances, at least 0.5% by weight surfactant is required.
The amount of agitation required to produce the acrylic polymers useful in the present invention depends on the composition of the polymerization medium. If a preferred surfactant is present in sufficient quantities to provide a stable emulsion of the polymer, moderate agitation is sufficient. However, more vigorous agitation is required with use of lesser amounts of surfactant or with use of a less efficient surfactant. A deficiency in agitation can be compensated for in part by an increase in mercaptan content. Likewise, increased agitation tends to reduce the amount of mercaptan required to provide a given molecular weight polymer, other factors being constant.
The polymerization preferably is carried out in the range of 25.degree.-65.degree. C. Use of relatively high temperatures increases the rate of polymerization while reducing the molecular weight of the acrylic polymer. Use of relatively low temperatures has the opposite effect. Use of temperatures below about 25.degree. C. results in polymerization rates too low to be commercially useful while temperatures above 65.degree. C. encourage inefficient initiator decomposition and increase side reactions between the initiator and the mercaptan chain transfer agent.
Polymer may be recovered from emulsions by freezing or coagulation of the latex with salts or acids. Preferably, excess monomers first are stripped off under vacuum to prevent further polymerization and to facilitate coagulation. Salts such as sodium chloride, aluminum sulfate or magnesium sulfate and acids such as hydrochloric, sulfuric or phorphoric acids are useful coagulants. After the coagulant is added to the stripped latex, the mixture is heated until the coagulated particles grow large enough to filter easily.
Alternatively, if suitable surfactants are used, the polymerization latex, after removal of unreacted monomers, can be flash extruded into plexifilaments without isolation of the polymer. Suitable surfactants for use in this direct polymerization and flash extrusion process include tridecylpoly(ethyleneoxy)phosphates such as "Gafac" RS 610 and "Gafac" RS 710; the nonylphenoxypoly(ethyleneoxy)sulfates such as "Alipal" EP 110 and "Alipal" EP 120; the nonylphenoxypoly(ethyleneoxy)phosphates such as "Gafac" RE 410, "Gafac" RE 610, "Gafac" RE 870 and "Gafac" PE 510; and dodecylbenzenesulfonates such as "Ultrawet" 89 LS. One skilled in the emulsifier art will recognize from this partial listing that many more of the commercially available surface-active agents will probably be satisfactory in the process of this invention.
In another modification of the invention, the dyesite and surfactant are combined in the form of a copolymer of acrylonitrile and 2-acrylamido-2-methylpropane sulfonic acid. About 2-3% by weight (based on monomers) of such a copolymer can be used as the surfactant in preparing an acrylonitrile/methyl acrylate copolymer suitable for use in the present invention. The dyesite/emulsifier copolymer becomes intimately and inseparably mixed with the acrylonitrile/methyl acrylate copolymer. The resulting latex has excellent stability and is especially suitable for optional direct flash extrusion without isolation of the polymer.
The process of this invention is a process for producing plexifilament strands which comprises in sequence:
(1) mixing water and an acrylonitrile polymer containing at least 91% by weight acrylonitrile groups and up to 9% by weight copolymeric units having an intrinsic viscosity of 0.6-2.0, 7 to 23 .mu.eq./g. enolizable groups after mild acid treatment, 1.5 to 7.0 .mu.eq./g. thioether ends derived from a water insoluble mercaptan and less than 3 .mu.eq./g. oxidizable hydrolysis fragments to obtain a substantially unform dispersion thereof, the polymer previously having either been isolated and washed or being in the emulsion form as prepared, the concentration of the polymer being between about 25% and 45% by weight based on the total weight of the dispersion, and adding up to 15% of a water-insoluble stabilizer based on polymer, if the polymer has been isolated and washed.
(2) heating the dispersion to a temperature between about 200.degree. C. and about 300.degree. C. but above the melting point of the complex formed by the polymer and water under at least autogeneous pressure, while maintaining the uniformity of dispersion, said heating occurring at a rate to minimize degradation of the polymer, the dispersion being held at least one or two minutes and
(3) extruding the dispersion abruptly into a region of substantially lower temperature and pressure.
The dispersion contains substantially molten acrylonitrile polymer complex with water as one phase and water as another phase under at least autogenous pressure at a temperature above the melting point of the acrylonitrile polymer/water-association complex. Use of water-insoluble nucleating agents in unnecessary when starting with a polymer emulsion prepared according to this invention.
If the dispersion is obtained by blending isolated and washed polymer with water in the desired amounts, along with the additives, the ingredients must be mixed well before heating to obtain a substantially uniform dispersion or slurry. A high-speed blender is suitable for this purpose.
Polymer concentration in the dispersion should be between about 25% and about 45% by weight based on the weight of the dispersion. A range of 30-45% is preferred. Above about 45% foam strands begin to be produced, and below about 25% a discontinuous "fly" or "fluff" begins to appear. In addition, when employing the water-insoluble inorganic oxide, its hydrate or salt thereof, it is preferred to use concentrations toward the higher end of their permissible range, e.g., 10-25% by weight based on the weight of the polymer, and preferably 10-15%, because lesser amounts tend to produce foams depending on the temperature and the concentration of polymer.
The temperature at which extrusion occurs and the rate at which the dispersion is heated are important factors. In general, the higher the extrusion temperature, the better the plexifilament strand produced, since the rapid "flashing" of liquid water into its gaseous phase is important to the successful production of the plexifilaments. The temperatures employed will range between about 200.degree. C. and 300.degree. C., with 240.degree. C. to 290.degree. C. preferred. However, the temperatures used, and the length of time taken to heat the dispersion up, both bear on the quality of the dispersion to be extruded, for the polymers in the heated water are susceptible to degradation. To minimize degradation in the continuous production of plexifilaments, the dispersion should be heated as rapidly as possible and should be extruded as soon as possible after reaching the desired extrusion temperature. Typically, time of heating does not exceed about 30 minutes. The temperature, of course, must be at least above the hydration temperature of the polymer used.
The pressure at which the dispersion is to be extruded must be at least autogenous pressure and preferably will range from about 500 to about 1500 psig (.about.3500-10,350 kPa).
It may sometimes be helpful to maintain the pH of the dispersion on the acidic side. For example, when a 93.8/6/0.1 acrylonitrile/methyl acrylate/sodium styrene sulfonate copolymer is used, the quality of the plexifilament strands produced is enhanced by using dispersions having a pH less than 5; while when a copolymer of 95/5 acrylonitrile/sodium styrene sulfonate is used a pH of less than 2 is desirable. The pH may be adjusted by adding an acid such as glacial acetic acid, sulfuric acid or the like.
The dispersion is maintained at the desired high temperature and pressure, then is abruptly extruded into a region of lower pressure and temperature, usually room temperature and pressure. The abrupt change in temperature and pressure causes the water to "flash", i.e., convert from liquid to gas, rapidly through the extrusion orifice, which in turn causes the formation of the plexifilament strands.
In order to maintain good uniformity of concentration in the dispersion, it is sometimes advantageous to employ a pressure let-down region, i.e., region of slightly reduced pressure, immediately adjacent the extrusion orifice to promote dispersibility just prior to extrusion.
The extrusion rate may range from 2000 yards per minute (ypm) or lower to 15,000 ypm or even higher depending on the pressure, viscosity of the dispersion and the size of the extrusion orifice. The orifice is a single orifice and may range from 0.005 "(0.127 mm) to about 0.1" (2.54 mm) in diameter.
The plexifilament strands produced by the process of this invention comprise a three-dimensional network of interconnected elements called fibrils. Usually the fibrils are less than 1.mu. in thickness and may be aggregated to larger fibrils of 5.mu. or less thickness. The fibrils may be thought of as an intermingled non-planar matrix of very thin film or ribbon-like elements that are irregularly interconnected (joined) at various points to form a web-like network or plexus.
The plexifilaments so produced are in the form of continuous strands (or yarns) and are characterized by high surface area, soft tactility and good cover. They are useful in the preparation of textile products, such as fabrics, tapes, ribbons, batts, and the like. Plexifilaments produced by the process of the present invention are water-wettable and rapidly absorb and transport water, thus making them particularly useful in towelling fabric uses.